In nerve and in muscle, action potentials are mediated by specialized membrane proteins in the plasma membrane of these cells. Although the function of these proteins, usually referred to as ion channels or excitable sites, has been well characterized electrophysiologically, the biochemistry of these proteins has hardly been studied. The goal of this research project is to study the biochemistry of one particular such protein, known as a sodium channel protein. This protein binds several neurotoxins, including saxitoxin and scorpion toxin, which can be radioactively labeled and used as specific molecular probes. During the course of cellular differentiation, myoblasts and neuroblasts, the developmental precursors of muscle and nerve cells, gradually acquire the proteins needed for electrical excitability. By following the biochemical events related to the acquisition of these proteins, we found the following. Based on detailed neurotoxin binding studies in muscle cells, we found that 125I-scorpion toxin binding activity develops earlier, faster, and reaches mature levels well before 3H-saxitoxin binding activity. Since the electrophysiological development parallels the slower 3H-saxitoxin binding activity, we proposed a model for the development of this protein. The models predicts that this protein gets inserted into the plasma membrane in an immature, electrophysiologically inactive form, capable of binding only 125I-scorpion toxin. During subsequent development, this protein undergoes a post-translational modification which renders it electrophysiologically active and capable of binding both 125I-scorpion and 3H-saxitoxin. We have solubilized and partially purified this protein from rat brain. By comparing the behavior of this protein from mature and immature rat brain on ion exchange and on lectin affinity columns, we found that the mature and immature forms are differentially glycosilated. This differential glycosilation may be the post-translation modification predicted by our hypothesis. We are currently further characterizing these differences and are raising monoclonal antibodies to the two different forms of this protein, as well as determining whether any of these developmental steps become impaired in disease.